Active matrix capacitive fingerprint sensor with 1-TFT pixel architecture for display integration

ABSTRACT

Embodiments described herein include a method for operating an input device by applying a charge voltage to a sense element through a first transistor that is between the sense element and a column output line and a first switch that is between the column output line and a drive voltage. The method also includes storing an electric charge on the sense element, wherein the electric charge comprises a magnitude corresponding to a feature of an input object. The method also includes driving a gate terminal of the first transistor low and disconnecting the charge voltage via the first switch. The method further includes transferring the electric charge to a feedback capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.14/788,499, titled “Active Matrix Capacitive Fingerprint Sensor with2-TFT Pixel Architecture for Display Integration,” and U.S. patentapplication Ser. No. 14/788,532, titled “Active Matrix CapacitiveFingerprint Sensor for Display Integration based on Charge Sensing by a2-TFT Pixel Architecture,” both filed concurrently herewith.

BACKGROUND

Field of the Disclosure

Embodiments of the present invention generally relate to a method andapparatus for touch sensing, and more specifically, to a fingerprintsensor.

Description of the Related Art

Input devices including proximity sensor devices (also commonly calledtouchpads or touch sensor devices) are widely used in a variety ofelectronic systems. Input devices also include fingerprint sensors andother biometric sensor devices. A sensor device typically includes asensing region, often demarked by a surface, in which the sensor devicedetermines the presence, location, motion, and/or features of one ormore input objects. Sensor devices may be used to provide interfaces forthe electronic system. For example, sensor devices are often used asinput devices for larger computing systems (such as opaque touchpads andfingerprint sensors integrated in, or peripheral to, notebook or desktopcomputers). Sensor devices are also often used in smaller computingsystems (such as touch screens integrated in cellular phones).

SUMMARY

Embodiments described herein include a processing system for operatingan input device and a method for operating an input device. In oneembodiment, a drive/readout circuit includes an amplifier circuitconnected to a feedback capacitor and a reset switch, a first switchconfigured to connect and disconnect a column output line to a drivevoltage, and a second switch configured to connect and disconnect thecolumn output line to the amplifier circuit. The drive/readout circuitis configured to apply the drive voltage to a sense element, bias thecolumn output line to ground while the sense element is disconnectedfrom the column output line, and read out a resulting signal from thesense element.

In another embodiment, an input device includes an array of sensingpixels configured to sense an input object in a sensing region. Each ofthe sensing pixels includes a sense element configured to store anelectric charge, wherein the electric charge includes a magnitudecorresponding to a feature of the input object. Each of the sensingpixels also includes a first transistor having a gate terminal connectedto a row select line, a second terminal connected to a column outputline, and a third terminal connected to the sense element. Adrive/readout circuit includes an amplifier circuit connected to afeedback capacitor, a reset switch, and a feedback switch. Thedrive/readout circuit is configured to accumulate a charge onto thefeedback capacitor over one or more charge and discharge cycles of thesense element using the feedback switch.

In another embodiment, a method for operating an input device includesapplying a charge voltage to a sense element through a first transistorthat is between the sense element and a column output line and a firstswitch that is between the column output line and a drive voltage. Themethod also includes storing an electric charge on the sense element,wherein the electric charge comprises a magnitude corresponding to afeature of an input object. The method also includes driving a gateterminal of the first transistor low and disconnecting the chargevoltage via the first switch. The method further includes transferringthe electric charge to a feedback capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a system that includes an input deviceaccording to an embodiment.

FIGS. 2A and 2B illustrate an example sensor electrode pattern andprocessing system according to one embodiment.

FIG. 3 illustrates a 1-TFT pixel architecture for an active matrixcapacitive fingerprint sensor according to one embodiment.

FIGS. 4A-4B illustrate schematics for a drive/readout circuit.

FIGS. 5A-5B illustrate timelines that comprises signal waveforms duringa drive/readout sequence.

FIGS. 6A-6D illustrate equivalent circuits of a pixel (i, j) connectedto a drive/readout circuit during charge, precharge, integrate, andreset stages.

FIGS. 6E-6G illustrate equivalent circuits of a pixel (i, j) connectedto a drive/readout circuit during charge, precharge, and read stages.

FIG. 7 illustrates a schematic for a drive/readout circuit.

FIG. 8 illustrates a timeline that comprises signal waveforms during acharge/precharge/integrate sequence.

FIG. 9 is a flowchart illustrating a method for operating an inputdevice according to one embodiment.

FIG. 10 illustrates a 2-TFT pixel architecture for an active matrixcapacitive fingerprint sensor according to another embodiment.

FIG. 11 illustrates a schematic for a drive/readout circuit.

FIG. 12 illustrates a schematic for a drive/readout circuit.

FIG. 13 illustrates a timeline that comprises signal waveforms during adrive/readout sequence.

FIGS. 14A-14C illustrate equivalent circuits of a pixel (i, j) connectedto a drive/readout circuit during enable, readout, and disable stages.

FIG. 15 illustrates signal waveforms and a drive circuit 1510 during adrive/readout sequence.

FIG. 16 illustrates a 2-TFT pixel architecture for an active matrixcapacitive fingerprint sensor according to another embodiment.

FIG. 17 illustrates a schematic for a drive/readout circuit.

FIG. 18 illustrates a schematic for a drive/readout circuit.

FIG. 19 illustrates a timeline that comprises signal waveforms during adrive/readout sequence.

FIGS. 20A-20C illustrate equivalent circuits of a pixel (i, j) connectedto a drive/readout circuit during enable, readout, and disable stages.

FIG. 21 illustrates signal waveforms and a drive circuit during adrive/readout sequence.

FIG. 22 is a flowchart illustrating a method for operating an inputdevice according to one embodiment.

FIG. 23 illustrates a 2-TFT pixel architecture for an active matrixcapacitive fingerprint sensor for display integration based on chargesensing according to another embodiment.

FIG. 24 illustrates a schematic for a drive/readout circuit.

FIG. 25 illustrates a schematic for a drive/readout circuit.

FIG. 26 illustrates a timeline that comprises signal waveforms during adrive/readout sequence.

FIGS. 27A and 27B illustrate equivalent circuits of a pixel (i, j)connected to a drive/readout circuit during enable, readout, and disablestages.

FIG. 28 illustrates a schematic for a drive/readout circuit.

FIG. 29 illustrates a schematic for a drive/readout circuit.

FIGS. 30A and 30B illustrate equivalent circuits of a pixel (i, j)connected to a drive/readout circuit during enable, readout, and disablestages.

FIG. 31 is a flowchart illustrating a method for operating an inputdevice according to one embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation. The drawings referred to here should not beunderstood as being drawn to scale unless specifically noted. Also, thedrawings are often simplified and details or components omitted forclarity of presentation and explanation. The drawings and discussionserve to explain principles discussed below, where like designationsdenote like elements.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the embodiments or the application and uses ofsuch embodiments. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

Various embodiments of the present technology provide input devices andmethods for improving usability. Particularly, embodiments describedherein provide a fingerprint sensor with increased sensor sensitivityand more accurate measurements. Embodiments also provide reducedthicknesses of layers of glass and layers of substrates. Embodimentsalso provide sensors with a small number of active elements, which mayreduce complexity and save space. Embodiments described herein may alsosubstantially nullify parasitic capacitances. Some embodiments integratea pixel charge over multiple charge and discharge cycles to make aninput signal easier to read. Fingerprint sensors described hereinprovide minimum impact on the optical performance of a display.Embodiments may reduce or cancel the effect of process variations acrossa pixel array. Some embodiments may provide a calibration process tocancel the effect of transistor performance variation and devicemismatch across the pixel array.

Turning now to the figures, FIG. 1 is a block diagram of an exemplaryinput device 100, in accordance with embodiments of the invention. Theinput device 100 may be configured to provide input to an electronicsystem (not shown). As used in this document, the term “electronicsystem” (or “electronic device”) broadly refers to any system capable ofelectronically processing information. Some non-limiting examples ofelectronic systems include personal computers of all sizes and shapes,such as desktop computers, laptop computers, netbook computers, tablets,web browsers, e-book readers, and personal digital assistants (PDAs).Additional example electronic systems include composite input devices,such as physical keyboards that include input device 100 and separatejoysticks or key switches. Further example electronic systems includeperipherals such as data input devices (including remote controls andmice), and data output devices (including display screens and printers).Other examples include remote terminals, kiosks, and video game machines(e.g., video game consoles, portable gaming devices, and the like).Other examples include communication devices (including cellular phones,such as smart phones), and media devices (including recorders, editors,and players such as televisions, set-top boxes, music players, digitalphoto frames, and digital cameras). Additionally, the electronic systemcould be a host or a slave to the input device.

The input device 100 can be implemented as a physical part of theelectronic system or can be physically separate from the electronicsystem. As appropriate, the input device 100 may communicate with partsof the electronic system using any one or more of the following: buses,networks, and other wired or wireless interconnections. Examples includeI²C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, and IRDA.

In FIG. 1, the input device 100 is shown as a proximity sensor device(also often referred to as a “touchpad” or a “touch sensor device”)configured to sense input provided by one or more input objects 140 in asensing region 120. Example input objects include fingers and styli, asshown in FIG. 1.

Sensing region 120 encompasses any space above, around, in, and/or nearthe input device 100 in which the input device 100 is able to detectuser input (e.g., user input provided by one or more input objects 140).The sizes, shapes, and locations of particular sensing regions may varywidely from embodiment to embodiment. In some embodiments, the sensingregion 120 extends from a surface of the input device 100 in one or moredirections into space until signal-to-noise ratios prevent sufficientlyaccurate object detection. The distance to which this sensing region 120extends in a particular direction, in various embodiments, may be on theorder of less than a millimeter, millimeters, centimeters, or more, andmay vary significantly with the type of sensing technology used and theaccuracy desired. Thus, some embodiments sense input that comprises nocontact with any surfaces of the input device 100, contact with an inputsurface (e.g., a touch surface) of the input device 100, contact with aninput surface of the input device 100 coupled with some amount ofapplied force or pressure, and/or a combination thereof. In variousembodiments, input surfaces may be provided by surfaces of casingswithin which the sensor electrodes reside, by face sheets applied overthe sensor electrodes or any casings, etc. In some embodiments, thesensing region 120 has a rectangular shape when projected onto an inputsurface of the input device 100.

The input device 100 may utilize any combination of sensor componentsand sensing technologies to detect user input in the sensing region 120.The input device 100 comprises one or more sensing elements fordetecting user input. As several non-limiting examples, the input device100 may use capacitive, elastive, resistive, inductive, magnetic,acoustic, ultrasonic, and/or optical techniques. Some implementationsare configured to provide images that span one, two, three, or higherdimensional spaces. Some implementations are configured to provideprojections of input along particular axes or planes. In some resistiveimplementations of the input device 100, a flexible and conductive firstlayer is separated by one or more spacer elements from a conductivesecond layer. During operation, one or more voltage gradients arecreated across the layers. Pressing the flexible first layer may deflectit sufficiently to create electrical contact between the layers,resulting in voltage outputs reflective of the point(s) of contactbetween the layers. These voltage outputs may be used to determinepositional information.

In some inductive implementations of the input device 100, one or moresensing elements pick up loop currents induced by a resonating coil orpair of coils. Some combination of the magnitude, phase, and frequencyof the currents may then be used to determine positional information.

In some capacitive implementations of the input device 100, voltage orcurrent is applied to create an electric field. Nearby input objectscause changes in the electric field and produce detectable changes incapacitive coupling that may be detected as changes in voltage, current,or the like.

Some capacitive implementations utilize arrays or other regular orirregular patterns of capacitive sensing elements to create electricfields. In some capacitive implementations, separate sensing elementsmay be ohmically shorted together to form larger sensor electrodes. Somecapacitive implementations utilize resistive sheets, which may beuniformly resistive.

Some capacitive implementations utilize “self capacitance” (or “absolutecapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes and an input object. In variousembodiments, an input object near the sensor electrodes alters theelectric field near the sensor electrodes, changing the measuredcapacitive coupling. In one implementation, an absolute capacitancesensing method operates by modulating sensor electrodes with respect toa reference voltage (e.g., system ground) and by detecting thecapacitive coupling between the sensor electrodes and input objects.

Some capacitive implementations utilize “mutual capacitance” (or“transcapacitance”) sensing methods based on changes in the capacitivecoupling between sensor electrodes. In various embodiments, an inputobject near the sensor electrodes alters the electric field between thesensor electrodes, changing the measured capacitive coupling. In oneimplementation, a transcapacitive sensing method operates by detectingthe capacitive coupling between one or more transmitter sensorelectrodes (also “transmitter electrodes” or “transmitters”) and one ormore receiver sensor electrodes (also “receiver electrodes” or“receivers”). Transmitter sensor electrodes may be modulated relative toa reference voltage (e.g., system ground) to transmit transmittersignals. Receiver sensor electrodes may be held substantially constantrelative to the reference voltage to facilitate receipt of resultingsignals. A resulting signal may comprise effect(s) corresponding to oneor more transmitter signals and/or to one or more sources ofenvironmental interference (e.g., other electromagnetic signals). Sensorelectrodes may be dedicated transmitters or receivers, or sensorelectrodes may be configured to both transmit and receive.Alternatively, the receiver electrodes may be modulated relative toground.

In FIG. 1, a processing system 110 is shown as part of the input device100. The processing system 110 is configured to operate the hardware ofthe input device 100 to detect input in the sensing region 120. Theprocessing system 110 comprises parts of, or all of, one or moreintegrated circuits (ICs) and/or other circuitry components. Forexample, a processing system for a mutual capacitance sensor device maycomprise transmitter circuitry configured to transmit signals withtransmitter sensor electrodes and/or receiver circuitry configured toreceive signals with receiver sensor electrodes. In some embodiments,the processing system 110 also comprises electronically-readableinstructions, such as firmware code, software code, and/or the like. Insome embodiments, components composing the processing system 110 arelocated together, such as near sensing element(s) of the input device100. In other embodiments, components of processing system 110 arephysically separate with one or more components close to sensingelement(s) of input device 100 and one or more components elsewhere. Forexample, the input device 100 may be a peripheral coupled to a desktopcomputer, and the processing system 110 may comprise software configuredto run on a central processing unit of the desktop computer and one ormore ICs (perhaps with associated firmware) separate from the centralprocessing unit. As another example, the input device 100 may bephysically integrated in a phone, and the processing system 110 maycomprise circuits and firmware that are part of a main processor of thephone. In some embodiments, the processing system 110 is dedicated toimplementing the input device 100. In other embodiments, the processingsystem 110 also performs other functions, such as operating displayscreens, driving haptic actuators, etc.

The processing system 110 may be implemented as a set of modules thathandle different functions of the processing system 110. Each module maycomprise circuitry that is a part of the processing system 110,firmware, software, or a combination thereof. In various embodiments,different combinations of modules may be used. Example modules includehardware operation modules for operating hardware such as sensorelectrodes and display screens, data processing modules for processingdata such as sensor signals and positional information, and reportingmodules for reporting information. Further example modules includesensor operation modules configured to operate sensing element(s) todetect input, identification modules configured to identify gesturessuch as mode changing gestures, and mode changing modules for changingoperation modes.

In some embodiments, the processing system 110 responds to user input(or lack of user input) in the sensing region 120 directly by causingone or more actions. Example actions include changing operation modes,as well as GUI actions such as cursor movement, selection, menunavigation, and other functions. In some embodiments, the processingsystem 110 provides information about the input (or lack of input) tosome part of the electronic system (e.g., to a central processing systemof the electronic system that is separate from the processing system110, if such a separate central processing system exists). In someembodiments, some part of the electronic system processes informationreceived from the processing system 110 to act on user input, such as tofacilitate a full range of actions, including mode changing actions andGUI actions.

For example, in some embodiments, the processing system 110 operates thesensing element(s) of the input device 100 to produce electrical signalsindicative of input (or lack of input) in the sensing region 120. Theprocessing system 110 may perform any appropriate amount of processingon the electrical signals in producing the information provided to theelectronic system. For example, the processing system 110 may digitizeanalog electrical signals obtained from the sensor electrodes. Asanother example, the processing system 110 may perform filtering orother signal conditioning. As yet another example, the processing system110 may subtract or otherwise account for a baseline, such that theinformation reflects a difference between the electrical signals and thebaseline. As yet further examples, the processing system 110 maydetermine positional information, recognize inputs as commands,recognize handwriting, and the like.

“Positional information” as used herein broadly encompasses absoluteposition, relative position, velocity, acceleration, and other types ofspatial information. Exemplary “zero-dimensional” positional informationincludes near/far or contact/no contact information. Exemplary“one-dimensional” positional information includes positions along anaxis. Exemplary “two-dimensional” positional information includesmotions in a plane. Exemplary “three-dimensional” positional informationincludes instantaneous or average velocities in space. Further examplesinclude other representations of spatial information. Historical dataregarding one or more types of positional information may also bedetermined and/or stored, including, for example, historical data thattracks position, motion, or instantaneous velocity over time.

In some embodiments, the input device 100 is implemented with additionalinput components that are operated by the processing system 110 or bysome other processing system. These additional input components mayprovide redundant functionality for input in the sensing region 120 orsome other functionality. FIG. 1 shows buttons 130 near the sensingregion 120 that can be used to facilitate selection of items using theinput device 100. Other types of additional input components includesliders, balls, wheels, switches, and the like. Conversely, in someembodiments, the input device 100 may be implemented with no other inputcomponents.

In some embodiments, the input device 100 comprises a touch screeninterface, and the sensing region 120 overlaps at least part of anactive area of a display screen. For example, the input device 100 maycomprise substantially transparent sensor electrodes overlaying thedisplay screen and provide a touch screen interface for the associatedelectronic system. The display screen may be any type of dynamic displaycapable of displaying a visual interface to a user, and may include anytype of light emitting diode (LED), organic LED (OLED), cathode ray tube(CRT), liquid crystal display (LCD), plasma, electroluminescence (EL),or other display technology. The input device 100 and the display screenmay share physical elements. For example, some embodiments may utilizesome of the same electrical components for displaying and sensing. Asanother example, the display screen may be operated in part or in totalby the processing system 110.

It should be understood that while many embodiments of the invention aredescribed in the context of a fully functioning apparatus, themechanisms of the present invention are capable of being distributed asa program product (e.g., software) in a variety of forms. For example,the mechanisms of the present invention may be implemented anddistributed as a software program on information bearing media that arereadable by electronic processors (e.g., non-transitorycomputer-readable and/or recordable/writable information bearing mediareadable by the processing system 110). Additionally, the embodiments ofthe present invention apply equally regardless of the particular type ofmedium used to carry out the distribution. Examples of non-transitory,electronically readable media include various discs, memory sticks,memory cards, memory modules, and the like. Electronically readablemedia may be based on flash, optical, magnetic, holographic, or anyother storage technology.

FIG. 2A shows a portion of an example sensor electrode patternconfigured to sense in a sensing region associated with the pattern,according to some embodiments. For clarity of illustration anddescription, FIG. 2A shows a pattern of simple rectangles, and does notshow various components. This sensor electrode pattern comprises aplurality of transmitter electrodes 160 (160-1, 160-2, 160-3, . . .160-n), and a plurality of receiver electrodes 170 (170-1, 170-2, 170-3,. . . 170-n) disposed over the plurality of transmitter electrodes 160.

Transmitter electrodes 160 and receiver electrodes 170 are typicallyohmically isolated from each other. That is, one or more insulatorsseparate transmitter electrodes 160 and receiver electrodes 170 andprevent them from electrically shorting to each other. In someembodiments, transmitter electrodes 160 and receiver electrodes 170 areseparated by insulative material disposed between them at cross-overareas; in such constructions, the transmitter electrodes 160 and/orreceiver electrodes 170 may be formed with jumpers connecting differentportions of the same electrode. In some embodiments, transmitterelectrodes 160 and receiver electrodes 170 are separated by one or morelayers of insulative material. In some other embodiments, transmitterelectrodes 160 and receiver electrodes 170 are separated by one or moresubstrates; for example, they may be disposed on opposite sides of thesame substrate, or on different substrates that are laminated together.

The areas of localized capacitive coupling between transmitterelectrodes 160 and receiver electrodes 170 may be termed “capacitivepixels.” The capacitive coupling between the transmitter electrodes 160and receiver electrodes 170 change with the proximity and motion ofinput objects in the sensing region associated with the transmitterelectrodes 160 and receiver electrodes 170.

In some embodiments, the sensor pattern is “scanned” to determine thesecapacitive couplings. That is, the transmitter electrodes 160 are drivento transmit transmitter signals. Transmitters may be operated such thatone transmitter electrode transmits at one time, or multiple transmitterelectrodes transmit at the same time. Where multiple transmitterelectrodes transmit simultaneously, these multiple transmitterelectrodes may transmit the same transmitter signal and effectivelyproduce an effectively larger transmitter electrode, or these multipletransmitter electrodes may transmit different transmitter signals. Forexample, multiple transmitter electrodes may transmit differenttransmitter signals according to one or more coding schemes that enabletheir combined effects on the resulting signals of receiver electrodes170 to be independently determined.

The receiver sensor electrodes 170 may be operated singly or multiply toacquire resulting signals. The resulting signals may be used todetermine measurements of the capacitive couplings at the capacitivepixels.

A set of measurements from the capacitive pixels form a “capacitiveimage” (also “capacitive frame”) representative of the capacitivecouplings at the pixels. Multiple capacitive images may be acquired overmultiple time periods, and differences between them used to deriveinformation about input in the sensing region. For example, successivecapacitive images acquired over successive periods of time can be usedto track the motion(s) of one or more input objects entering, exiting,and within the sensing region.

The background capacitance of a sensor device is the capacitive imageassociated with no input object in the sensing region. The backgroundcapacitance changes with the environment and operating conditions, andmay be estimated in various ways. For example, some embodiments take“baseline images” when no input object is determined to be in thesensing region, and use those baseline images as estimates of theirbackground capacitances.

Capacitive images can be adjusted for the background capacitance of thesensor device for more efficient processing. Some embodiments accomplishthis by “baselining” measurements of the capacitive couplings at thecapacitive pixels to produce a “baselined capacitive image.” That is,some embodiments compare the measurements forming a capacitance imagewith appropriate “baseline values” of a “baseline image” associated withthose pixels, and determine changes from that baseline image.

In some embodiments, transmitter electrodes 160 comprise one or morecommon electrodes (e.g., “V-com electrode”) used in updating the displayof the display screen. These common electrodes may be disposed on anappropriate display screen substrate. For example, the common electrodesmay be disposed on the TFT glass in some display screens (e.g., InplaneSwitching (IPS) or Plane to Line Switching (PLS)), on the bottom of thecolor filter glass of some display screens (e.g., Patterned VerticalAlignment (PVA) or Multi-domain Vertical Alignment (MVA)), etc. In suchembodiments, the common electrode can also be referred to as a“combination electrode”, since it performs multiple functions. Invarious embodiments, each transmitter electrode 160 comprises one ormore common electrodes. In other embodiments, at least two transmitterelectrodes 160 may share at least one common electrode.

In various embodiments, the “capacitive frame rate” (the rate at whichsuccessive capacitive images are acquired) may be the same or bedifferent from that of the “display frame rate” (the rate at which thedisplay image is updated, including refreshing the screen to redisplaythe same image). In some embodiments where the two rates differ,successive capacitive images are acquired at different display updatingstates, and the different display updating states may affect thecapacitive images that are acquired. That is, display updating affects,in particular, the background capacitive image. Thus, if a firstcapacitive image is acquired when the display updating is at a firststate, and a second capacitive image is acquired when the displayupdating is at a second state, the first and second capacitive imagesmay differ due to differences in the background capacitive imageassociated with the display updating states, and not due to changes inthe sensing region. This is more likely where the capacitive sensing anddisplay updating electrodes are in close proximity to each other, orwhen they are shared (e.g. combination electrodes).

For convenience of explanation, a capacitive image that is taken duringa particular display updating state is considered to be of a particularframe type. That is, a particular frame type is associated with amapping of a particular capacitive sensing sequence with a particulardisplay sequence. Thus, a first capacitive image taken during a firstdisplay updating state is considered to be of a first frame type, asecond capacitive image taken during a second display updating state isconsidered to be of a second frame type, a third capacitive image takenduring a third display updating state is considered to be of a thirdframe type, and so on. Where the relationship of display update stateand capacitive image acquisition is periodic, capacitive images acquiredcycle through the frame types and then repeats.

FIG. 2B illustrates a system 200 for sensing an input object accordingto embodiments of the present disclosure. System 200 comprises an array210 of sensing pixels in a sensing region 120, each sensing pixelcomprising a sense element 220. Sense elements in the embodimentsdescribed below could comprise sense plates or any other passive oractive elements. Sense elements are operable to determine a feature oreffect of an input object. For example, a sense element that is a senseplate could determine a capacitance between the sense plate and an inputobject, such as a finger. This capacitance can then determine a portionof a fingerprint pattern. The array of sensing elements may alsocomprise the electrodes described above with respect to FIG. 2A.

Processing system 110 in FIG. 2B is operable to transmit and receivesignals to and from array 210. The processing system 110 may include adriver module 230, a receiver module 240, a determination module 250,and an optional memory 260. The receiver module 240 is coupled to thearray 210 and configured to receive resulting signals indicative ofinput (or lack of input) in the sensing region 120 and/or ofenvironmental interference. The receiver module 240 may also beconfigured to pass the resulting signals to the determination module 250for determining the presence of an input object (such as a finger)and/or to the optional memory 260 for storage. In various embodiments,integrated circuits in the processing system 110 may be coupled todrivers for sending signals to array 210. The drivers may be fabricatedusing thin-film-transistors (TFT) and may comprise switches,combinatorial logic, multiplexers, and other selection and controllogic.

The driver module 230, which includes driver circuitry, included in theprocessing system 110 may be configured for sending signals to array210. The driver module 230 may send signals that set row select, enable,or supply lines high or low, as described in further detail below. Thedriver module 230 may produce signals that turn switches on or off asdescribed in further detail below. Processing system 110 may beimplemented with more circuitry to control the various componentsdescribed in the example embodiments below.

Embodiments described below comprise fingerprint sensors utilizingthin-film transistors (TFTs). Fingerprint sensors can be incorporatedinto a display in certain embodiments. For a fingerprint sensorincorporated into a display, the fingerprint sensor elements may beincorporated near a top surface of the display to improve the quality ofa signal captured to detect fine features of a fingerprint. The senseelements for fingerprint sensing described below could be incorporatedin an entire active area of a display, or in only a part of the activearea of the display. The sense elements may have a pixel density thatmatches the pixel density of the display pixels, in which case a senseelement could be incorporated in every pixel of the display, or in everypixel of the relevant portion of the active area configured forfingerprint sensing. The sense elements could also have a pixel densitythat is greater or less than the pixel density of the display, and ifthe sensing pixels have a pixel density greater than the display thenmultiple sense elements of the fingerprint sensor could be incorporatedin a single display pixel. Input device 100, described above withrespect to FIG. 1, comprises a fingerprint sensor in certain embodimentsdescribed herein.

Fingerprint sensors detect the valleys and ridges of fingerprints. Onetechnique for detecting a fingerprint comprises detecting changes insensor capacitances along valleys and ridges of the fingerprint to getan image of the fingerprint, which may be all or a portion of thecomplete fingerprint pattern of a user's finger. A cover layer may beemployed over the fingerprint sensor to protect the sensor. The coverlayer can protect display elements and/or proximity sensor elements inaddition to fingerprint sensor elements. The cover layer may be made ofan opaque material, or a transparent material, such as glass. This coverlayer may be 500 microns or less in some embodiments. With fingerprintsensing, capacitances may be measured on the order of 10⁻¹⁸ F. A valleydepth in a fingerprint may be approximately 60 microns. Ridge-to-ridgespacing may be approximately 400 microns. A thickness of a ridge may be100-300 microns. Therefore a pixel size for a fingerprint sensor ofaround 40-70 microns on a side may be sufficient to capture ridge andvalley information of a fingerprint. A pixel pitch of around 40-70microns may also be sufficient to capture ridge and valley informationof a fingerprint. Pixel pitch may be 20-100 microns in some embodiments.Smaller pixel sizes and/or pixel pitches can be used to capture smallerfeatures, such as sweat pores, in addition to ridge and valleyinformation of a fingerprint.

With many sensor pixels, it is difficult to place 6 or more TFTs foreach pixel to operate a fingerprint sensor due to space constraints.Embodiments described below can work with as few as 1 TFT or 2 TFTs foreach sensing pixel. The architectures described below could be discreteor incorporated in a display. In addition, architectures described belowcan produce waveforms large enough to nullify parasitic capacitances.

Operational amplifiers described in embodiments below can be low voltageintegrated circuits or may be embodied on a panel. Switches describedbelow may be in an integrated circuit or embodied on a non-conductivesupporting substrate, such as glass or plastic. MEMS(micro-electro-mechanical) switches may be utilized and may be formed ona supporting substrate or in an integrated circuit. Switches andtransistors may be formed in semiconductor wafers or may be TFTs. Senseelements in the embodiments described below could comprise sense plates,PN diodes, piezoelectric transducers that sense ultrasonic waves, or anypassive or active elements that accumulate a charge or transduce anexcitation into a charge in the presence of an input object, such as afinger.

Embodiments described below that sense a capacitance associated with aninput object may measure absolute capacitance or trans capacitance.Absolute capacitance measures a capacitance between the input object anda sense element. Trans capacitance measures a change in capacitancebetween two sense electrodes due to the presence of an input object.

Embodiments described below may integrate a charge over multiple cyclesto more easily capture the fingerprint.

Features described in separate embodiments below may be combined,removed, or incorporated into the other embodiments where appropriate.

Active Matrix Capacitive Fingerprint Sensor with 1-TFT PixelArchitecture for Display Integration

FIG. 3 illustrates a pixel architecture for an active matrix capacitivefingerprint sensor according to one embodiment. Architecture 300 mayoperate with as few as one TFT in each sensing pixel. Architecture 300comprises an array 310 of sense elements 302 (in this example, the senseelements 302 comprise sense plates 302) each addressed through a selectthin-film transistor (TFT) 304 controlled by a row of addressing lines(row select 306). Each column of sense plates 302 is connected to acommon output line 308. When a row is selected, sense plate 302 of eachcolumn is connected to the common output line 308 of that column throughthe respective TFT 304. The TFT 304 may be in-cell if the sensor isintegrated in a display.

The array 310 of architecture 300 may use as few as one TFT per pixel,one output line per column, and one address line per row which reducesthe impact on the optical performance of the display. An externalcircuit (described in further detail below) comprising four switches, afeedback capacitance, and a high gain operational amplifier providescancellation of the parasitic capacitance of the output line. Inaddition, integration of the pixel charge can be performed over multiplecharge and discharge cycles in some embodiments.

FIG. 4A illustrates a schematic for a drive/readout circuit 400 of acolumn j connected to sense plate 402 at row 406 _(i) and column 408.Select TFT 404 is coupled to row select line 406 _(i) and output line408. Drive/readout circuit 400 also comprises four switches: S_(1j) 412,S_(2j) 414, S_(F) 418, and S_(R) 420. The feedback network comprisesfeedback capacitance C_(F) 422 and reset switch S_(R) 420, and theamplifier circuit comprises operational amplifier 416. Switch S_(1j) 412charges the sense plate 402 by coupling the plate to V_(ch) 410 throughselect TFT 404. Switch S_(2j) 414 is utilized for readout of the storedcharge on sense plate 402. Feedback switch S_(F) 418 connects anddisconnects the feedback capacitance C_(F) 422 to an input of theoperational amplifier 416. Reset switch S_(R) 420 resets the state ofdrive/readout circuit 400 between subsequent readout of the rows i.Feedback capacitance C_(F) 422 provides feedback to operationalamplifier 416, which has one input coupled to ground 426. In someembodiments a clock signal may be coupled to an input terminal of theoperational amplifier 416.

FIG. 5A illustrates timeline 500 that comprises signal waveforms duringthe drive/readout sequence in accordance with FIGS. 3 and 4A. A 3-stepsequence is used to transfer the charge on the capacitance formedbetween the sense plate 402 and a finger to the feedback capacitanceC_(F) 422. This capacitance contains the information related to thetopography of the finger surface. The charge can be integrated duringmultiple charge/discharge cycles to increase the amplitude of the outputsignal by repeating the 3-step sequence. FIG. 5A illustrates thewaveforms of the row select 406 _(i) and control signal of the switchesS_(1j), S_(2j), S_(F), and S_(R). At time T1, sense plate 402 isconnected to the charge voltage V_(ch) 410 through the select TFT 404and switch S_(1j) 412, i.e. row select 406 _(i) and S_(1j) signals areset to High. Meanwhile S_(2j) 414 and feedback switch S_(F) 418 remainopen. Reset switch S_(R) 420 remains closed. As shown, S_(2j) 414 andS_(F) 418 are Low and S_(R) 420 is High. During this time (chargestage), charge is stored on sense plate 402 with a magnitudeproportional to the capacitance to the finger.

At time T2, the TFT 404 is disconnected from the output line 408 byturning Row select 406 _(i) to Low. S_(1j) 412 is opened (S_(1j) 412 isturned Low) to disconnect the charge voltage V_(ch) 410.

At time T3 (output pre-charge stage), S_(2j) 414 is closed (S_(2j) 414is turned High) to pre-charge the output line to virtual ground (in thecase of a non-ideal operational amplifier, to the input offset voltageof the operational amplifier V_(os)).

At time T4, S_(R) 420 is opened (S_(R) 420 is turned Low). At Time T5,S_(F) 418 is closed to configure the circuit for readout of the storedcharge. At time T6 (Integrate stage), Row select 406 _(i) is closed totransfer the charge to C_(F) 422 and consequently change the outputvoltage 424 to a value proportional to the stored charge on the senseplate 402.

At time T7, S_(F) 418 is opened (S_(F) 418 is turned Low) to disconnectthe feedback capacitance C_(F) 422 from the operational amplifier 416and retain the charge on C_(F) 422. At time T8, the circuit can enteranother charging stage by connecting the charge voltage V_(ch) 410through the select TFT 404 and switch S_(1j) 412; i.e. row select 406_(i) and S_(1j) 412 signals are set to High. Meanwhile, S_(2j) 414 isopened, S_(R) 420 is closed, and S_(F) 418 remains open (S_(2j) 414turns Low and S_(R) 420 turns High). By completing anothercharge/precharge/integrate cycle, the pixel charge can be added to(integrated on) the feedback capacitor 422. At the end of the N^(th)cycle, the output voltage 424 can be sampled and the output can be resetby turning on the S_(R) switch 420. At time TR1, the S_(F) 418 is opened(S_(F) 418 turns Low) to initialize the circuit for another readoutsequence.

FIGS. 6A-6D illustrate equivalent circuits of a pixel (i, j) connectedto the drive/readout circuit of FIG. 4A during charge, precharge,integrate, and reset stages. The sense plate 402 capacitance to thefinger is denoted by C_(in) and the parasitic capacitances of the outputline are lumped into the capacitance C_(p). FIG. 6A illustrates theequivalent circuit 610 during a charge stage (T1<t<T2 as illustrated inFIG. 5A). FIG. 6B illustrates the equivalent circuit 620 during aprecharge stage (T3<t<T4 as illustrated in FIG. 5A). FIG. 6C illustratesthe equivalent circuit 630 during an integrate stage (T6<t<T7 asillustrated in FIG. 5A). FIG. 6D illustrates the equivalent circuit 640during a reset stage (T7N<t<TR1 as illustrated in FIG. 5A). Isolation ofthe readout circuit from the charge voltage V_(ch) using switch S_(2j)allows the readout circuit, including the operational amplifier andreset switch S_(R), to be implemented using lower voltage technologythan the drive circuit.

At the end of each charge stage, the sense plate 402 voltage isV_(in)=V_(ch) and the negative terminal of the operational amplifier 416V−=V_(out)=0 (or equals V_(os) close to 0). A charge of Q_(in)=C_(in)V_(ch) is accumulated on sense plate 402. This charge is retained onsense plate 402 by turning off the TFT 404 at the end of the chargestage. During the pre-charge charge stage, the output line 408 isisolated from the power supply and connected to the input of theoperational amplifier 416. At the end of the pre-charge stage, thevoltage of the output line V_(ij)=V⁻=V_(out)=0 (or equals V_(os) closeto 0), and the charge stored on C_(F) 422 is zero. At the end of thefirst read stage, the voltage of the output line V_(i)=V_(in)=V⁻,V_(out)=AV⁻, and −V_(CF)=V_(out)−V⁻=(A−1)V⁻. If the gain of operationalamplifier 416 is large enough, the charge transferred to the parasiticcapacitance C_(p) during the readout of the sense capacitor isnegligible compared to the charge transferred to C_(F), as the voltageof C_(p) does not change during the readout. Hence, the effect of theparasitic capacitance is cancelled. The S_(F) 418 is closed during theintegration stage to allow charge to be accumulated on the feedbackcapacitor C_(F) 422, while S_(F) 418 is open during charge and prechargestages. The S_(F) 418 and S_(R) 420 are closed in the reset stage todischarge the feedback capacitor C_(F) 422 and reset the output voltage424.

FIGS. 4B, 5B, and 6E-6G are schematic diagrams illustrating anotherembodiment of a drive/readout circuit. The embodiment illustrated inFIG. 4B is similar to FIG. 4A with the exception of the removal of thefeedback switch in FIG. 4B. FIGS. 5B and 6E-6G are also associated withthe embodiment of FIG. 4B. FIG. 4B illustrates the schematic of thedrive/readout circuit 450 of the column j connected to the sense plateat row i and column j. The readout circuit includes 3 switches (S_(1j)412, S_(2j) 414, and S_(R) 420), an operational amplifier 416, and afeedback capacitance C_(F) 422. Switch S_(1j) 412 is used for chargingthe plate, switch S_(2j) 414 is used for readout of the stored charge onthe sense plate, switch S_(R) 420 is used to reset the state of thecircuit between subsequent readout of the rows, and C_(F) provides thefeedback to the operational amplifier.

FIG. 5B illustrates timeline 550 that comprises signal waveforms duringthe drive/readout sequence in accordance with FIGS. 3 and 4B. A 3 stepsequence is used to measure the capacitance formed between the senseplate and a finger; this capacitance contains the information related tothe topography of the finger surface. FIG. 5B shows the waveforms of therow select (i) and control signal of the switches S_(1j), S_(2j), andS_(R).

At time T₁, the sense plate is connected to the charge voltage V_(Ch)through the select TFT and switch S_(1j); i.e. row select (i) and S_(1j)signals are set to High. Meanwhile S_(2j) remains open and S_(R) remainsclosed (S_(2j) is Low and S_(R) is High). This disconnects the senseplate from the readout circuit and resets the output voltage bydischarging the charge stored on feedback capacitance C_(F). During thistime (the charge stage), charge is stored on the sense plate with amagnitude proportional to the capacitance to the finger.

At time T₂, the TFT is disconnected from the output line by turning Rowselect (i) to Low and S_(1j) is opened (S_(1j) is turned Low) todisconnect the charge voltage. At time T₃ (output pre-charge stage),S_(2j) is closed (S_(2j) is turned High) to pre-charge the output lineto virtual ground (in the case of a non-ideal op-amp to the input offsetvoltage of the op-amp V_(os)). At time T₄, S_(R) is opened (S_(R) isturned Low) to configure the circuit for readout of the stored charge.At time T5 (Read stage), Row select (i) is closed to transfer the chargeto C_(F) and consequently change the output voltage to a valueproportional to the stored charge on the sense plate.

FIGS. 6E-6G illustrate equivalent circuits of a pixel (i, j) connectedto the drive/readout circuit of FIG. 4B during charge, precharge, andread stages. The sense plate 402 capacitance to the finger is denoted byC_(in) and the parasitic capacitances of the output line are lumped intothe capacitance C_(p). FIG. 6E illustrates the equivalent circuit 650during a charge stage (T1<t<T2 as illustrated in FIG. 5B). FIG. 6Fillustrates the equivalent circuit 660 during a precharge stage (T3<t<T4as illustrated in FIG. 5B). FIG. 6G illustrates the equivalent circuit670 during a read stage (T5<t<T6 as illustrated in FIG. 5B).

With respect to FIGS. 4B, 5B, and 6E-6G, at the end of the charge stage,the plate voltage is V_(in)=V_(ch) and the negative terminal of theop-amp V⁻=V_(out)=0 (or V_(os) close to 0). A charge ofQ_(in)=C_(in)V_(ch) is accumulated on the sense plate; this charge isretained on the sense plate by turning off the TFT at the end of thecharge stage. During the pre-charge stage, the output line is isolatedfrom the power supply and connected to the input of the operationalamplifier. At the end of the pre-charge stage, the voltage of the outputline V_(ij)=V⁻=V_(out)=0 (or V_(os) close to 0) and the charge stored onC_(F) is zero. At the end of the read stage, the V_(ij)=V_(in)=V⁻,V_(out)=AV⁻, and −V_(CF)=V_(out)−V⁻=(A−1) V⁻. For the case of largeenough gain of the operational amplifier, the charge transferred to theparasitic capacitance C_(p) during the readout of the sense capacitor isnegligible compared to the charge transferred to C_(F) as the voltage ofC_(p) does not change during the readout. Hence the effect of theparasitic capacitance is cancelled.

For a first case, (infinite gain (A) and zero V_(os)): V⁻=V_(1j)=0, asthe gain is infinite and the offset voltage is zero. Therefore, thecharge stored on the sense plate is transferred to C_(F).

$V_{out} = {{V_{-} - V_{CF}} = {{- \frac{Q_{in}}{C_{F}}} = {{- \frac{C_{in}}{C_{F}}}V_{ch}}}}$

For a second case of a non-ideal operational amplifier:

  V_(out) = V⁻ − V_(CF)   V_(out) = −A(V⁻ − V_(os)) + V_(cs)$\mspace{20mu}{V_{CF} = \frac{\left\lbrack {{C_{in}\left( {V_{oh} - V_{-}} \right)} - {C_{p}\left( {V_{-} - V_{OS}} \right)}} \right\rbrack}{C_{F}}}$$V_{out} = {{{- \left\lbrack {1 + {{1/A}\frac{C_{in} + C_{p} + C_{F}}{C_{F}}}} \right\rbrack^{- 1}}\frac{C_{in}}{C_{F}}V_{oh}} + {\left\lbrack {1 + {{1/A}\frac{C_{in} + C_{p} + C_{F}}{C_{F}}}} \right\rbrack^{- 1}\frac{A + 1}{A}\frac{1}{C_{F}}\left( {C_{in} + C_{F} + {C_{p}/A}} \right)V_{OS}}}$

To minimize the effect of C_(p) on the output voltage:

${{1/A}\frac{C_{in} + C_{p} + C_{F}}{C_{F}}} ⪡ 1$

In many practical cases, C_(p)>>C_(F)>>C_(in), simplifying the conditionto:

$\frac{C_{p}}{C_{F}} ⪡ A$

Under this condition:

$V_{out} = {{{- \frac{C_{in}}{C_{F}}}V_{oh}} + {\frac{C_{in} + C_{F}}{C_{F}}V_{os}}}$

From this equation, the effect of the offset voltage can be neglected ifV_(ch)>>V_(os).

FIG. 7 illustrates a schematic for a drive/readout circuit 700 of acolumn j connected to sense plate 702 at row 706 _(i) and column 708_(j). FIG. 7 is similar to FIG. 4A, with the addition of switch S_(3j)728 added to each column for precharging the output line to ground 726.In the embodiments of FIGS. 4-6, the pre-charge state is implemented byconnecting the output line to virtual ground through the switch S_(2j)414 and reset switch S_(R) 420. In FIG. 7, switch S_(3j) 728 allows thepre-charge state to instead be implemented by connecting the output line708 directly to system ground 726. Switch S_(3j) 728 can be implementedusing a TFT on a display/sensor backplane or using a transistor in adriver circuit. Select TFT 704 is coupled to row select line 706, andoutput line 708 _(j). Drive/readout circuit 700 comprises four otherswitches: S_(1j) 712, S_(2j) 714, feedback switch S_(F) 718, and resetswitch S_(R) 720. Drive/readout circuit 700 further comprises feedbackcapacitance C_(F) 722 and operational amplifier 716.

Implementation of the pre-charge switch S_(3j) 728 on the backplaneallows the charge integrator to be isolated from the high voltage builtup on the output line 708 _(j) during the charging step. This allowsimplementation of charge integrator using a low-voltage technology forbetter performance and smaller chip footprint. This also allows for adecrease in the time needed for the pre-charge phase, as the pre-chargeswitch S_(3j) 728 can have a higher limit on current than a limit in theoperational amplifier circuits.

After the charge stage and isolation of the sense plate 702 using selectTFT 704, the output line 708 is biased to ground 726 using thepre-charge switch S_(3j) 728. Next, the switch S_(2j) 714 is closed andthe output line is connected to the input stage of the integrator. Atthis stage, select TFT 704 is opened to transfer the charge to thefeedback capacitance 722. As the line parasitic capacitance is orders ofmagnitude larger than the sense plate 702 capacitance, the chargeintegrator is only exposed to a very small transient voltage. Hence,charging voltages with magnitudes substantially larger than theoperating voltage rating of the charge integrator circuit can beemployed.

Alternatively, both charging and pre-charging biases can be applied tothe output line through S_(1j) 712 using a signal with a properwaveform.

FIG. 8 illustrates timeline 800 that comprises signal waveforms duringthe charge/precharge/integrate sequence in accordance with FIG. 7. Thewaveforms are similar to timeline 500 illustrated in FIG. 5A anddescribed in detail above. Timeline 800 introduces the waveform S_(3j)for switch 728. Switch S_(3j) 728 is asserted High during the prechargestage at time T3. Switch S_(3j) 728 is then asserted Low at time T4.

FIG. 9 is a flowchart illustrating a method 900 for operating an inputdevice, according to one embodiment. The steps of method 900 may beperformed in any suitable order. The method begins at step 910, where adriver module applies a charge voltage to a sense element through afirst transistor and a first switch. The driver module may also set arow select high at this step. At step 920, an electric charge is storedon the sense element. The electric charge comprises a magnitudeproportional to a feature of an input object. This feature may be acapacitance associated with the input object. The feature may be acapacitance between the sense element and the input object. If a fingeris the input object and a fingerprint is being sensed, the magnitude ofthe capacitance is measured to determine the depth of a ridge or valleyof a fingerprint.

At step 930, a gate terminal of the first transistor is driven low andthe first switch is opened by the driver module to disconnect the chargevoltage. The gate terminal can be driven low by driving the row selectline to low. At step 940, the charge voltage is transferred to afeedback capacitor. After the charge is transferred to the feedbackcapacitor, the charge can be read with a readout circuit, or additionalcycles may be performed to integrate additional charge on the feedbackcapacitor before the charge is read out. After the charge is read out,the circuit can be initialized for another drive/readout sequence.

Active Matrix Capacitive Fingerprint Sensor with 2-TFT PixelArchitecture for Display Integration

FIG. 10 illustrates a pixel architecture for an active matrix capacitivefingerprint sensor according to one embodiment. Architecture 1000 mayoperate with as few as two TFTs in each sensing pixel. Architecture 1000comprises an array 1020 of sense elements 1002 (in this example senseelements 1002 comprise sense plates 1002) each addressed through a TFTcircuit 1004 controlled by a row of addressing lines (row select 1006)and a row of enable lines 1012. Each TFT circuit 1004 is connected to acommon output line 1008 and to a supply line 1010.

FIG. 11 illustrates a schematic of pixel circuit 1100 of a column jconnected to sense plate 1102 at row 1106 _(i) and column 1108 _(j).Each sense electrode is connected through a first TFT T1 _(i,j) 1112 toan enable line 1110. The first TFT 1112 T1 _(i,j) is controlled by a rowselect line 1106 coupled to a gate electrode. Each sense plate 1102 isconnected to the gate of a second TFT T_(2i,j) 1116 while the drain ofthe second TFT T_(2i,j) 1116 is connected to the supply line 1104 andits source is connected to the output line 1108. The reference capacitorC_(R) 1114 is connected between the gate and source of the second TFT1116. Each row of pixels shares the same enable line 1110 and row selectline 1106, and all pixels in the same column share the same supply line1104 and output line 1108. In a variation of the pixel architecturediscussed in further detail below, no supply line 1104 is included andthe drain of the second TFT T_(2i,j) 1116 is connected to the row selectline 1106 (see, e.g., FIG. 17). In this sensor, the capacitance formedbetween the sense plate 1102 and surface of the finger controls thesteady-state output current of the second TFT T_(2i,j) 1116. Bymeasuring the output current of the pixel, the capacitance between thesense plate 1102 and the finger can be determined for each pixel,thereby providing an image of the finger surface.

The architecture illustrated in FIGS. 10 and 11 provides a minimumimpact on optical performance of the display, as the architecture uses 2TFTs per pixel with small dimensions. As the steady state current of thepixel represents the value of the capacitance between the finger and thesense plate 1102 (which is determined by the shape of the fingersurface), the measurement time of the output current can be increased toenhance the accuracy of the measurement.

Operation of the second TFT T_(2i,j) 1116 in the sub-threshold regime ispossible to benefit from an exponential current-voltage dependence(i.e., the current has an exponential dependence to the value of thefinger-sense plate capacitance). To the first order, the parasiticelements do not impact the response of the sensor output as the sensoroperates in steady-state mode.

The circuit can be operated in a three-stage drive/readout sequence toextract the TFT IV characteristics for accurate calculation of thefinger capacitance. This method cancels the effect of process variationresulting in characteristic mismatch across the array. It is alsopossible to calibrate the device by scanning the array when no finger ispresent to cancel the effect of TFT performance variation and devicemismatch across the array.

FIG. 12 illustrates a schematic 1200 of a drive/readout circuit of thecolumn j connected to the pixel at row i and column j for the structureillustrated in FIGS. 10 and 11. The readout circuit includes twoswitches S_(1j) 1232 and S_(2j) 1234, an operational amplifier 1224, anda feedback resistor R_(F) 1222. Switch S_(1j) 1232 is used to connectthe output to a first bias voltage −V_(Bias1) 1228, and switch S_(2j)1234 is used to connect the output to the second bias voltage −V_(Bias2)1230. Schematic 1200 further comprises TFTs 1212 and 1216 andcapacitances C_(R) 1214 and C_(in i,j) 1218 (the input object is assumedto be coupled to ground 1220). Feedback resistor R_(F) 1222 is coupledto amplifier 1224 and V_(out) 1226. Row select line 1206, enable line1210, supply 1204, and output 1208 are also illustrated in FIG. 12.

FIG. 13 illustrates timeline 1300 that comprises signal waveforms duringthe drive/readout sequence in accordance with FIGS. 10, 11, and 12. A3-step sequence is used to measure the capacitance formed between thesense plate 1102 and a finger. This capacitance contains the informationrelated to the topography of the finger surface. The readout sequenceconsists of an enable step, readout step, and a disable step. To enablethe pixel, at time T1, sense plate 1102 is connected to the enable line1210 through the TFT T_(1i,j) 1212; i.e., Row Select 1206 _(i) is set toHigh and enable 1210 _(i) is biased at 0 V. The Supply 1204 _(j) is alsoset to V_(dd). This will set the potential of sense plate 1102 to 0 V.During this time switch S_(1j) 1228 is High (closed or connected) andswitch S_(2j) 1234 is Low (open or disconnected). As a result the outputvoltage is at −V_(Bias1)−RI_(Sense1). I_(Sense1) is a function of IVcharacteristics of T_(2i,j) 1216 and V_(Bias1) 1228. It is important tonote that I_(Sense1) is independent of the absolute or trans capacitanceof the input object. During the readout step, at time T2, sense plate1102 is isolated from enable line 1210; i.e., Row Select 1206 _(i) isset to Low (0 or a negative voltage) and enable 1210 _(i) is biased at−V_(SS). Next, at time T3, switch S_(1j) 1232 is set to Low and switchS_(2j) 1234 is set to High. This connects V_(Bias2) 1230 to the positiveterminal of the operational amplifier 1224 and isolates V_(Bias1) 1228from the operational amplifier 1224. For an op-amp with large enoughgain, the voltage of the negative terminal of the op-amp becomes−V_(Bias2); hence the Output (j) 1208 is pulled down to −V_(Bias2) (from−V_(Bias1)). As a result, the output current of the T_(2i,j) 1216changes and the V_(out) will change to −V_(Bias2)−RI_(Sense2), whereI_(Sense2) is a function of the measured capacitance (either absolute ortrans capacitance) and characteristics of the TFT 1216. At time T4(start of the Disable step), T_(2i,j) 1216 is turned Off by biasing thegate of T_(2i,j) 1216 at −V_(SS) by setting the row select line 1206_(i) to V_(dd) and the enable line 1210 _(i) to −V_(SS). This will setthe voltage of V_(out) 1226 and output line 1208 _(j) to −V_(Bias2). Attime T5, switch S_(1j) 1232 is set to High and switch S_(2j) 1234 toLow, to reset the voltage of output line 1208 _(j) and V_(out) 1226 to−V_(Bias1). Finally, at time T6, the Disable stage is finalized bysetting the row select 1206 _(i) to 0 V. At this point the pixel isready for the next Enable/Readout/Disable sequence.

FIGS. 14A-14C illustrate equivalent circuits of a pixel (i, j) connectedto the drive/readout circuit during enable, readout, and disable stages,respectively. The sense plate 1102 capacitance to the finger (absolutecapacitance) is denoted by C_(in) and it is assumed that the parasiticgate-source capacitance of T2 _(i,j) 1216 is included in C_(R), and therest of the parasitic elements are ignored. FIG. 14A illustrates theequivalent circuit 1410 during an enable stage (T1<t<T2 as illustratedin FIG. 13). FIG. 14B illustrates the equivalent circuit 1420 during areadout stage (T3<t<T4 as illustrated in FIG. 13). FIG. 14C illustratesthe equivalent circuit 1430 during a disable stage (T5<t<T6 asillustrated in FIG. 13).

FIG. 15 illustrates signal waveforms 1500 and drive circuit 1510 duringthe drive/readout sequence for the pixel architecture of FIGS. 10-12implemented without switch S_(1j) and switch S_(2j). As the state ofswitches S_(1j) and S_(2j) are opposite (illustrated in waveform 1500),it is possible to remove both switches and apply the proper signaldirectly to the positive terminal of operational amplifier 1224 as shownin FIG. 15.

FIG. 16 illustrates a 2-TFT pixel architecture for an active matrixcapacitive fingerprint sensor according to another embodiment.Architecture 1600 comprises an array 1620 of sense elements (senseelements 1602 comprise sense plates 1602 in this embodiment) eachaddressed through a TFT circuit 1604 controlled by a row of addressinglines (row select 1610) and a row of enable lines 1612. Each TFT circuit1604 is connected to a common output line 1608. In this architecture, noseparate supply line is included and the drain of the second TFT iscoupled to the row select line (compare to architecture 1000 in FIG.10).

FIG. 17 illustrates a schematic for a pixel circuit 1700 of a column jconnected to sense plate 1702 at row i 1704. Each sense electrode isconnected through a first TFT T_(1i,j) 1712 to an Enable line 1710. Thefirst TFT T_(1i,j) 1712 is controlled by a row select/supply line 1704coupled to a gate electrode of TFT T_(1i,j) 1712 (no separate supplyline is included in this embodiment). Each sense plate 1702 is connectedto the gate of a second TFT T_(2i,j) 1716 while the drain of the secondTFT T_(2i,j) 1716 is connected to the row select/supply line 1704 andits source is connected to the output line 1708. The referencecapacitance C_(R) 1714 is connected between the gate and source of thesecond TFT T_(2i,j) 1716. The drain of the second TFT T_(2i,j) 1716 isconnected to the row select/supply line 1704. In this schematic, thecapacitance formed between the sense plate 1702 and surface of thefinger controls the steady-state output current of the second TFTT_(2i,j) 1716. By measuring the output current of the pixel, thecapacitance between the sense plate 1702 and a finger can be determined,thereby providing an image of the finger surface.

FIG. 18 illustrates a schematic 1800 of a drive/readout circuit of thecolumn j connected to the pixel at row i and column j for the structureillustrated in FIGS. 16 and 17. The readout circuit includes twoswitches S_(1j) 1832 and S_(2j) 1834, an operational amplifier 1824, anda feedback resistor R_(F) 1822. Switch S_(1j) 1832 is used to connectthe output to a first bias voltage −V_(Bias1) 1828, and switch S_(2j)1834 is used to connect the output to the second bias voltage −V_(Bias2)1830. Schematic 1800 further comprises TFTs 1812 and 1816 and capacitorsC_(R) 1814 and Ci_(n i,j) 1818 (the input object is assumed to becoupled to ground 1820). Feedback resistor R_(F) 1822 is coupled toamplifier 1824 and V_(out) 1826. Row select/supply 1806, enable 1810,and output 1808 are also illustrated in FIG. 18.

FIG. 19 illustrates timeline 1900 that comprises signal waveforms duringthe drive/readout sequence in accordance with FIGS. 16, 17, and 18. A3-step sequence is used to measure the capacitance formed between thesense plate 1702 and an input object, such as a finger. This capacitancecontains the information related to the topography of the fingersurface. The readout sequence consists of an enable step, readout step,and a disable step. The waveforms are similar to the waveforms discussedabove with respect to FIGS. 11-13, except that there are no supply linesfor FIGS. 16-18, and the select/supply lines replace the row selectlines. For a detailed discussion of the operations, see FIG. 13 above.

FIGS. 20A-20C illustrate equivalent circuits of a pixel (i, j) connectedto the drive/readout circuit during enable, readout, and disable stages,respectively. The capacitance between sense plate 1702 and the finger isdenoted by C_(in) and it is assumed that the parasitic gate-sourcecapacitance of T_(2i,j) 1816 is included in C_(R), and the rest of theparasitic elements are ignored. FIG. 20A illustrates the equivalentcircuit 2010 during an enable stage (T1<t<T2 as illustrated in FIG. 19).FIG. 20B illustrates the equivalent circuit 2020 during a readout stage(T3<t<T4 as illustrated in FIG. 19). FIG. 20C illustrates the equivalentcircuit 2030 during a disable stage (T5<t<T6 as illustrated in FIG. 19).

FIG. 21 illustrates signal waveforms 2100 and drive circuit 2110 duringthe drive/readout sequence for the pixel architecture of FIGS. 16-18implemented without switches S_(1j) and S_(2j). Because the switchesS_(1j) and S_(2j) are driven opposite one another (when one is high, theother is low), it is possible to remove both switches and apply theappropriate V_(Bias1) or V_(Bias2) signal directly to the positiveterminal of operational amplifier 1824 as shown in FIG. 21.

With respect to both 2-TFT architectures illustrated in FIGS. 10-12 and16-18, during the enable stage, the potential of the gate of the sensetransistor (T_(2i,j)) is raised to 0 V. This allows for a current flowin this transistor when a negative voltage is applied to the source ofthis transistor. During the readout stage, the source voltage of sensetransistor changes from −V_(Bias1) to −V_(Bias2). Initially (enablestage), the output current of the sense transistor is independent of thecapacitance detected by the sense element, but at the later stage theoutput current will be a function of this capacitance (see equationsbelow). Hence, in the initial stage (enable stage), the sense transistorcan be characterized and the sense capacitor can be accuratelydetermined in the second stage. This will eliminate the effect ofprocess variation and device mismatch across the array. During theDisable stage, a −V_(SS) potential is applied to the gate of the sensetransistor to ensure that the TFT remains in the off state when the restof the pixels in the same column are addressed. The following providesthe equations for sense current and output voltage during the readout ofa pixel. It is assumed that the circuit has reached steady statecondition. The current of the TFT is a function of V_(DS) and V_(GS)expressed as f(V_(GS2), V_(DS2)).

Equations for the fingerprint sensor of FIGS. 10-12:

At T₃ ⁻ (just before changing the state of S_(1j) and S_(2j)):V _(GS2)=0−(−V _(Bias1))=V _(Bias1)V _(DS2) =V _(dd)−(−V _(Bias1))=V _(dd) +V _(Bias1)I _(Sense1) =f(V _(Bias1) ,V _(dd) +V _(Bias1))V _(out) =−V _(Bias1) −RI _(Sense1)

At T₄ ⁻ (just before changing the state of row select (i)):

$V_{{GS}\; 2} = {{{- \frac{C_{R}\left( {V_{{Bias}\; 2} - V_{{Bias}\; 1}} \right)}{C_{R} + C_{in}}} - \left( {- V_{{Bias}\; 2}} \right)} = \frac{{C_{in}V_{{Bias}\; 2}} + {C_{R}V_{{Bias}\; 1}}}{C_{R} + C_{in}}}$V_(DS 2) = V_(dd) − (−V_(Bias 2)) = V_(dd) + V_(Bias 2)$I_{{Sense}\; 1} = {f\left( {\frac{{C_{in}V_{{Bias}\; 2}} + {C_{R}V_{{Bias}\; 1}}}{C_{R} + C_{in}},{V_{dd} + V_{{Bias}\; 2}}} \right)}$V_(out) = −V_(Bias 2) − RI_(Sense 2)

Equations for the fingerprint sensor of FIGS. 16-18:

At T₃ ⁻ (just before changing the state of S_(1j) and S_(2j))V _(GS2)=0−(−V _(Bias1))=V _(Bias1)V _(DS2)=0−(−V _(Bias1))=V _(Bias1)I _(Sense1) =f(V _(Bias1) ,V _(Bias1))V _(out) =−V _(Bias1) −RI _(Sense1)

At T₄ ⁻ (just before changing the state of Row Select (i))

$V_{{GS}\; 2} = {{{- \frac{C_{R}\left( {V_{{Bias}\; 2} - V_{{Bias}\; 1}} \right)}{C_{R}❘C_{in}}} - \left( {- V_{{Bias}\; 2}} \right)} = \frac{{C_{in}V_{{Bias}\; 2}} + {C_{R}V_{{Bias}\; 1}}}{C_{R}❘C_{in}}}$V_(DS 2) = 0 − (−V_(Bias 2)) = V_(Bias 2)$I_{{Sense}\; 1} = {f\left( {\frac{{C_{in}V_{{Bias}\; 2}} + {C_{R}V_{{Bias}\; 1}}}{C_{R} + C_{in}},V_{{Bias}\; 2}} \right)}$V_(out) = −V_(Bias 2) − RI_(Sense 2)

Assuming the TFT operates in subthreshold regime with I∝e^(KV) ^(GS)dependence:

${I_{{Sense}\; 1} = {A\mspace{11mu}{\exp\left( \frac{{BC}_{in} + {DC}_{R}}{C_{R} + C_{in}} \right)}}},$

where A, B, and D are constants and a function of TFT characteristics,V_(Bias1), and V_(Bias2). The current therefore has an exponentialrelationship to the input capacitance, and a small change in the inputcapacitance can produce a large variation in sense current.

Although in the 2-TFT example architectures above, the−V_(Bias1)>−V_(Bias2), it is possible to run the embodiments in thecondition where −V_(Bias2)>−V_(Bias1). Also, the reference capacitanceC_(R) may be implemented via an additional reference capacitor connectedto the two terminals of the second TFT T_(2i,j) transistor, or the gateto source capacitance of the second TFT may be sufficient.

In the 2-TFT example architectures above, it is possible to read theoutput current only once by applying a voltage pulse to the positiveterminal. A calibration step can be applied occasionally to determinethe IV characteristics of the sense TFTs across the array. Theseparameters can be stored and used to avoid measuring the current twotimes in the same frame. Under this condition, the V_(Bias) signal (seeFIG. 15 and FIG. 21) changes from 0 to −V_(Bias) and the current is onlymeasured at T₄ ⁻.

FIG. 22 is a flowchart illustrating a method 2200 for operating an inputdevice, according to one embodiment. The steps of method 2200 may beperformed in any suitable order. Method 2200 describes anenable/readout/disable sequence for a fingerprint sensor with a 2-TFTpixel architecture. The method begins at step 2210, where a drivermodule asserts a row select line high to set a voltage at a senseelement to zero. The row select line is coupled to a gate terminal of afirst transistor, and a second terminal of the first transistor iscoupled to the sense element. A third terminal of the first transistoris coupled to an enable line.

At step 2220, the driver module asserts the row select line low and theenable line is biased to a negative voltage. This step isolates thesense element from the enable line. At step 2230, an output current issensed on a second terminal of a second transistor. third terminal ofthe second transistor may be coupled to a supply line, or to a combinedselect/supply line in some embodiments. The output current isproportional to a feature of the input object. For example, the outputcurrent may be proportional to a capacitance between the input object(such as a finger) and the sense element. The output current cantherefore be used to determine an image of a fingerprint pattern, whichmay be all or a portion of a complete fingerprint of a user.

Active Matrix Capacitive Fingerprint Sensor for Display Integrationbased on Charge Sensing by a 2-TFT Pixel Architecture

FIG. 23 illustrates a pixel architecture for an active matrix capacitivefingerprint sensor for display integration based on charge sensingaccording to one embodiment. Architecture 2300 may operate with as fewas two TFTs, or one TFT and one diode in each sensing pixel.Architecture 2300 comprises an array 2320 of sense elements (senseelements 2302 comprise sense plates 2302 in this embodiment) eachaddressed through a TFT circuit 2304 controlled by a row of addressinglines (row select 2310) and a row of enable lines 2312. Each TFT circuit2304 is connected to a common output line 2308.

FIG. 24 illustrates a schematic of a pixel 2400 of a column j connectedto sense plate 2402 at row 2404 _(i) and column 2408 _(j). Referencecapacitor C_(R) 2414 may employed in some embodiments. Each sense plate2402 is connected through the first TFT T_(1i,j) 2412 to an enable line2410 and the first TFT T_(1i,j) 2412 is controlled by a row select line2404. Each sense plate 2402 is connected to the gate and drain of asecond TFT T_(2i,j) 2416 while the source of TFT T_(2i,j) 2416 isconnected to the output line 2408 (the second TFT is diode-connected tocreate a two terminal device). The reference capacitor C_(R) 2414 (ifused) is connected between the gate and source of the second TFTT_(2i,j) 2416. Each row of pixels share the same enable line 2410 androw select line 2404, and all pixels in the same column share the sameoutput line 2408. In a variation of the pixel architecture discussed infurther detail below, the second TFT T_(2i,j) 2416 is replaced by adiode or other non-linear circuit element (see FIGS. 28-30). In thisarchitecture, the charge stored on the sense plate 2402 is measured todetermine the capacitance between the sense plate 2402 and the finger,hence providing an image of the finger surface.

The architecture illustrated in FIGS. 23 and 24 provides a minimumimpact on optical performance of the display as the sensor may use asfew as two TFTs (or one diode and one TFT) per pixel with smallestpossible dimensions. Parasitic capacitance of the output line may beeffectively cancelled and produces no artifact on the measured charge asthe voltage of the output line remains constant during the enable andreadout stages.

The steady state current flowing through the sense transistor (secondTFT T_(2i,j) 2416) can be used to measure the IV characteristics of thedevice to cancel the effect of TFT characteristics mismatch across thearray. Finally, it is possible to calibrate the device by scanning thearray when no finger is present to cancel the effect of TFT performancevariation and device mismatch across the array.

FIG. 25 illustrates a schematic 2500 of a drive/readout circuit of thecolumn j connected to the pixel at row i and column j for the pixelstructure illustrated in FIG. 23. The readout circuit includes a switchS_(Rj) 2522, an operational amplifier 2524, and a feedback capacitorC_(F) 2526. Switch S_(Rj) 2522 is used to reset the charge stored onfeedback capacitor C_(F) 2526 between consecutive readouts. Schematic2500 further comprises TFTs T_(1i,j) 2512 and T_(2i,j) 2516 andcapacitances C_(R) 2514 and C_(in i,j) 2518 (input capacitance, coupledto ground 2520). Feedback capacitor C_(F) 2526 and switch S_(Rj) 2522are coupled to operational amplifier 2524 and V_(out) 2530. Row select2506, enable 2510, and output 2508 are also illustrated in FIG. 25.

FIG. 26 illustrates timeline 2600 that comprises signal waveforms duringthe drive/readout sequence in accordance with FIGS. 23, 24, and 25. A3-step sequence is used to determine the capacitance formed between thesense plate 2402 and a finger by measuring the charge stored on senseplate 2402 due to this capacitance. This capacitance represents theinformation related to the topography of the finger surface. FIG. 26shows the waveforms of the lines for pixels of 24 and 25. The readoutsequence consists of an Enable step, Readout step, and a Disable step.To enable the pixel, at time T₁, the sense plate 2402 is connected tothe enable line 2510 _(i) through the TFT T_(1i,j) 2512; i.e., rowselect 2506 _(i) is set to High and enable 2510 _(i) is biased atV_(dd). During this time, switch S_(Rj) 2522 is closed (switch S_(Rj) isHigh) so V_(out) is held at ground as the positive terminal of theoperational amplifier 2524 is grounded at 2528 and the output isconnected to the negative terminal. In this step, the current followingthrough the TFT T_(2i,j) 2516 is only a function of the TFTcharacteristics, and may be measured for calibration purposes.

At time T₂, Row Select 2506 _(i) and Enable 2510 _(i) lines areconnected to ground and switch S_(Rj) 2522 is opened (switch S_(Rj) 2522is turned Low). This step isolates the sense plate 2402 from the Enableline 2510 _(i) and transfers the charge stored on the sense plate 2402(shown as C_(in i,j) 2518) into feedback capacitor C_(F) 2526.Consequently, V_(in i,j) drops to a value below the threshold voltage ofTFT T_(2i,j) 2516, and V_(out) drops to a negative value depending onthe stored charge according to the equations presented below.

At time T₃, row select 2506 _(i) is connected to V_(dd) to turn on TFTT_(1i,j) 2516, and switch S_(Rj) 2522 is closed (switch S_(Rj) 2522turns High). Hence, the pixel is disabled by setting the voltage ofV_(in i,j) to 0 V to eliminate the charge leakage through TFT T_(2i,j)2516 during the readout of the pixels in other rows. The V_(out) is alsoset to 0 V by discharging the feedback capacitor C_(F) 2526.

At time T₄, row select line 2506 _(i) is set to 0 V to prepare the pixelfor another Enable/Readout/Disable sequence. To increase the speed ofsensing an input capacitance, it is possible to combine the enable stepof the row (i+1) with the disable step of the row (i).

FIGS. 27A and 27B illustrate equivalent circuits of a pixel (i, j)connected to the drive/readout circuit during enable, readout, anddisable stages. The capacitance between sense plate 2402 and a finger isdenoted as C_(in i,j) and it is assumed that the parasitic gate-sourcecapacitance of the TFT T_(2i,j) 2516 is included in C_(R), and the restof the parasitic elements are ignored. FIG. 27A illustrates theequivalent circuit 2710 during the enable or disable stage (T1<t<T2 andT3<t<T4 as illustrated in FIG. 26). FIG. 27B illustrates the equivalentcircuit 2720 during a readout stage (T2<t<T3 as illustrated in FIG. 26).

FIG. 28 illustrates a schematic for a drive/readout circuit 2800 of acolumn j connected to sense plate 2802 at row 2804 _(i) and column 2808.Drive/readout circuit 2800 is identical to circuit 2400 illustrated inFIG. 24 with the exception of the non-linear circuit element (rectifyingelement) comprising the second TFT T_(2i,j) 2416 being replaced by anon-linear circuit element (rectifying element) comprising a diodeD_(i,j) 2816. The structure and operation of the circuits are similar.In FIGS. 24 and 28, like numerals denote like elements (i.e., senseplate 2402 is equivalent to sense plate 2802, etc.). The operation andadvantages described above with respect to FIGS. 23-27 also apply toFIGS. 28-30.

FIG. 29 illustrates a schematic 2900 of a drive/readout circuit of thecolumn j connected to the pixel at row i and column j for the pixelstructure illustrated in FIG. 28. Schematic 2900 is identical toschematic 2500 illustrated in FIG. 25 with the exception of TFT 2516being replaced by diode 2916. The structure and operation of thecircuits are similar. In FIGS. 25 and 29, like numerals denote likeelements (i.e., C_(F) 2526 is equivalent to C_(F) 2926, etc.).

FIGS. 30A and 30B illustrate equivalent circuits of a pixel (i, j)connected to the drive/readout circuit during enable, readout, anddisable stages. Equivalent circuit 3010 is identical to equivalentcircuit 2710 illustrated in FIG. 27 with the exception of TFT T_(2i,j)being replaced by diode D_(i,j). The structure and operation of thecircuits are similar.

With respect to both the 2-transistor structure of FIGS. 24-27 and thetransistor-plus-diode structure of FIGS. 28-30, during the enable stage,the potential of the gate of the sense transistor (T_(2i,j)) or theanode terminal of the diode (D_(i,j)) (i.e., the non-linear circuitelement) is raised to V_(dd). This acts to store a charge on the senseplate proportional to a measured capacitance (either absolutecapacitance or trans capacitance). A constant current also followsthrough the transistor or diode (i.e., the non-linear circuit element),which is only a function of the IV characteristics of the device and canbe measured to calibrate the sensor to cancel the effect of devicemismatch across the pixel array. During the readout, the sense plate isisolated from the enable line and the charge stored on the sense plateis transferred to C_(F). As the output current remains constant, nomeasured to calibrate the sensor to cancel the effect of device mismatchacross the pixel array. During the readout, the sense plate is isolatedfrom the enable line and the charge stored on the sense plate istransferred to C_(F). As the output current remains constant, no chargeis transferred to the output line parasitic capacitance. This willeliminate the effect of the parasitic elements of the output line.During the Disable stage, the sense TFT or diode is turned off bysetting the voltage of the sense plate to 0 V. Hence, the pixel remainsin the off state when the rest of the pixels in the same column areaddressed. The following provide the equations for V_(out) during thereadout of the pixel. It is assumed that the diode or TFT stopsconducting at V_(T) or V_(ON).

During Enable and Disable steps:V _(out)=0 V

At T₃ ⁻ (just before changing the state of row select (i)):

$V_{out} = {{- \frac{\left( {C_{{{in}\mspace{11mu} i},j} + C_{R}} \right)\left( {V_{dd} - V_{T}} \right)}{C_{F}}}\mspace{14mu}{for}\mspace{14mu}{{FIG}.\mspace{11mu} 24}}$$V_{out} = {{- \frac{\left( {C_{{{in}\mspace{11mu} i},j} + C_{R}} \right)\left( {V_{dd} - V_{ON}} \right)}{C_{F}}}\mspace{14mu}{for}\mspace{14mu}{{FIG}.\mspace{11mu} 28}}$

With respect to both the 2-transistor structure of FIGS. 24-27 and thetransistor-plus-diode structure of FIGS. 28-30, it is possible toconnect the positive terminal of the operational amplifier to anarbitrary bias voltage (for example −V_(Bias)) to increase the storedcharge over the pixel capacitor by increasing the bias voltage fromV_(dd) to V_(dd) V_(Bias) across the pixel capacitor. Under thiscondition, the Enable line should be biased at −V_(Bias) during thedisable stage.

A calibration step can be applied occasionally to determine the IVcharacteristics of the sense TFT or diode across the array by measuringthe current flowing through the device during the Enable step.

begins at step 3110, where a driver module asserts a row select linehigh to couple a sense element to an enable line through a firsttransistor. The row select line is coupled to a gate terminal of thefirst transistor, and a first terminal of the first transistor iscoupled to the sense element. The enable line is coupled to a secondterminal of the first transistor.

At step 3120, a charge is collected at the sense element, where thecharge is proportional to a feature of an input object. The charge maybe proportional to a capacitance between an input object (such as afinger) and the sense element. At step 3130, the driver module assertsthe row select line and the enable line low to isolate the sense elementfrom the enable line. At step 3140, the charge stored on the senseelement is transferred to a feedback capacitor (as a result of step3130). The charge is transferred through a non-linear circuit element.The non-linear circuit element may be a diode or a transistor-connecteddiode.

At step 3150, an output voltage is read. The output voltage isproportional to the feature of the input object, and may be used todetermine at least a portion of a fingerprint. After the output voltagehas been read, the pixel may be reset to prepare for anotherenable/readout/disable sequence.

Thus, the embodiments and examples set forth herein were presented inorder to best explain the embodiments in accordance with the presenttechnology and its particular application and to thereby enable thoseskilled in the art to make and use the invention. However, those skilledin the art will recognize that the foregoing description and exampleshave been presented for the purposes of illustration and example only.The description as set forth is not intended to be exhaustive or tolimit the invention to the precise form disclosed.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

What is claimed is:
 1. An input device, comprising: a sensing pixelconfigured to sense an input object in a sensing region, wherein thesensing pixel comprises a sense element configured to store an electriccharge, wherein the electric charge comprises a magnitude correspondingto a feature of the input object; and a drive/readout circuitcomprising: an amplifier circuit connected to a feedback capacitor and areset switch; a first switch configured to connect and disconnect acolumn output line to a drive voltage; and a second switch configured toconnect and disconnect the column output line to the amplifier circuit;wherein the drive/readout circuit is configured to: apply the drivevoltage to the sense element; bias the column output line to groundwhile the sense element is disconnected from the column output line; andread out a resulting signal from the sense element.
 2. The input deviceof claim 1, wherein the resulting signal corresponds to a capacitanceformed between the sense element and the input object in the sensingregion.
 3. The input device of claim 1, wherein the drive/readoutcircuit further comprises a feedback switch, wherein the feedbackcapacitor is configured to accumulate a charge over one or more chargeand discharge cycles of the sense element using the feedback switch. 4.The input device of claim 3, wherein the feedback switch is furtherconfigured to connect and disconnect the feedback capacitor to an inputof the amplifier circuit.
 5. The input device of claim 1, wherein thedrive/readout circuit further comprises a third switch configured toconnect and disconnect the column output line to system ground, whereinthe drive/readout circuit is configured to bias the column output lineto ground by closing the third switch.
 6. A method for operating aninput device, comprising: applying a drive voltage to a sense elementvia a first switch configured to connect and disconnect a column outputline; biasing the column output line to ground while the sense elementis disconnected from the column output line via a second switch; andreading out a resulting signal from the sense element.
 7. The method ofclaim 6, wherein the resulting signal corresponds to a capacitanceformed between the sense element and an input object in a sensingregion.
 8. The method of claim 6, wherein a feedback capacitor isconfigured to accumulate a charge over one or more charge and dischargecycles of the sense element using a feedback switch.
 9. The method ofclaim 8, wherein the feedback switch is further configured to connectand disconnect the feedback capacitor to an input of an amplifiercircuit.
 10. A processing system for operating an input device,comprising: a drive/readout circuit comprising: an amplifier circuitconnected to a feedback capacitor and a reset switch; a first switchconfigured to connect and disconnect a column output line to a drivevoltage; and a second switch configured to connect and disconnect thecolumn output line to the amplifier circuit; wherein the drive/readoutcircuit is configured to: apply the drive voltage to a sense element;bias the column output line to ground while the sense element isdisconnected from the column output line; and read out a resultingsignal from the sense element.
 11. The processing system of claim 10,wherein the resulting signal corresponds to a capacitance formed betweenthe sense element and an input object in a sensing region.
 12. Theprocessing system of claim 10, wherein the drive/readout circuit furthercomprises a feedback switch, wherein the feedback capacitor isconfigured to accumulate a charge over one or more charge and dischargecycles of the sense element using the feedback switch.
 13. Theprocessing system of claim 12, wherein the feedback switch is furtherconfigured to connect and disconnect the feedback capacitor to an inputof the amplifier circuit.
 14. The processing system of claim 10, whereinthe drive/readout circuit further comprises a third switch configured toconnect and disconnect the column output line to system ground, whereinthe drive/readout circuit is configured to bias the column output lineto ground by closing the third switch.
 15. The processing system ofclaim 10, wherein the drive/readout circuit is configured to bias thecolumn output line to ground by closing the second switch and the resetswitch to bias the column output line to virtual ground.
 16. Theprocessing system of claim 10, wherein the amplifier circuit is a lowervoltage circuit than the drive voltage.
 17. The processing system ofclaim 10, wherein the drive/readout circuit is configured to address thesense element through a first transistor, wherein the first transistorhas a gate terminal connected to a row select line, a second terminalconnected to the column output line, and a third terminal connected tothe sense element.
 18. The processing system of claim 17, wherein thefirst transistor is a thin-film transistor.
 19. The processing system ofclaim 10, wherein the sense element has a pitch suitable fordistinguishing between features of a fingerprint.
 20. The processingsystem of claim 19, wherein the pitch comprises a range of 20 to 100microns.